The present invention relates to a security alarm system incorporating a radio communication link between remote sensors and a central security monitoring device.
It has long been considered desirable in an alarm system to use a radio communication link between the remote sensors, such as door switches, window switches, manually-operable personal emergency signal switches, and the like, and a central security monitoring and alarm-signaling device. The potential advantages of such a system include avoiding the expense of installing wiring between remote switches and the central monitoring device, invulnerability of a radio-linked system to cutting of linking wiring, and the possibility of using a portable central security monitoring device.
The disadvantages of previously available wireless systems, however, have made such systems extremely costly, undependable, or both. While it is possible to reduce the cost of such systems by using a single radio transmission frequency for all of the sensor-transmitter units in a system, use of a single radio frequency present problems of mutual interference between the several transmitters of a system. This problem is potentially increased where there are several similar systems operating in proximity with one another on the same frequency, as is possible within a large office building.
Another problem in previously known wireless alarm systems has been the absence of a transmitted signal so long as there is no unauthorized passage through an alarm-equipped door or window. The radio transmitters associated with the door switches or other sensors in such a system are ordinarily battery-powered, to leave the system operable in the event of commercial power failure, and transmission is minimized to extend battery life. The lack of a continuous positive indication that each particular transmitter is operable has resulted in a general lack of faith in such radio-linked alarm systems in the past. Although it would be possible to continuously transmit signals indicating a normal or safe condition, this would rapidly drain the battery supply power of a remote transmitter in such a system. Additionally, FCC regulations limit the amount of time during which such systems are permitted to transmit, making continuous transmission illegal. One solution to this problem, therefore, has been the use of periodic transmissions of limited duration from each of the transmitters in an alarm system. Mutual radio interference is a problem inherent in such a method of operation of an alarm system, however, when a single transmission frequency is used for all of the transmitter units of the system. Avoiding such interference by using a multiplicity of transmission frequencies increases the cost of a receiver portion of the system. Use of a schedule system to avoid simultaneous transmissions by the various transmitters of a system also adds to the cost of the system.
A separate, but related disadvantage of previously known radio-linked alarm systems has been the necessity to compromise between the desired small transmitter size which would permit convenient inconspicuous mounting of transmitter units in, for example, a window frame or doorway frame, and the desirability of extending the length of time during which the transmitters will remain operable without battery replacement.
Yet another problem which must be addressed in development of a radio-linked alarm system is that it is desirable to know which of several sensors has detected an intrusion and originated an alarm signal transmission. This particular problem has been addressed previously in several ways, including pulse-count identification codes, transmitter identification by transmitted tone modulation, and by digital identification codes transmitted either by transmission pulse techniques or frequency shift keying. As previously employed, however, none of these techniques avoids the problem of mutual radio frequency interference among several transmitters all operating on the same nominal frequency for reception by a single receiver.
Yet a further problem associated with radio-linked systems is that it may be difficult or impossible to economically produce transmitters whose output frequencies are precisely the same and stable enough through long periods of time to assure reception by a radio receiver portion of a central alarm control device whose reception bandwidth is narrow enough to reduce background radio frequency noise in order to provide an adequate signal/noise ratio for reliable reception.
What is needed, then, is a radio-linked security alarm system which is low in cost, provides an extended period of operability without maintenance, whose transmitters are small enough for convenient installation, in which frequent reassurance of the operability of each transmitter is provided, and wherein positive identification of the location of an intrusion is provided automatically by the transmitter unit.